1. Field of the Invention
The invention relates to an optical film with super low retardation and its application in polarizing plates.
2. Description of the Related Art
Liquid crystal displays (LCDs) are widely used in personal computers, digital cameras, cell phones, and televisions. Polarizer is an essential component of LCDs, which polarizes the incident light from random polarization into a specific polarization direction. LCDs are usually operated between two crossed polarizers, such that by controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts, correspondingly illuminating the pixel. There is an increasing demand for polarizers due to the growing LCD market.
The polarization function of the polarizer is obtained by orienting a light anisotropic absorber in a transparent polymer film. For example, a PVA-based polarizer can be obtained by allowing iodine or a dichromatic dye to be absorbed on polyvinyl alcohol (PVA) film, followed by uniaxial orientation by stretching.
As application fields of LCDs become wider, and durability requirements for polarizers become more severe as a result, polarizer protective films are commonly provided on both sides of the polarizer to improve durability and mechanical strength. Polarizer protective films also prevent dissociation of iodine ion of the polarizer in humid conditions. Triacetyl cellulose (TAC) has been the dominating material for polarizer protective films due to its high transparency, optically uniform appearance, and easy adhesion to polarizer.
Large-size LCDs have become increasingly popular in recent years. Along with such trends, lowering the birefringence of polarizer protective films is required for the optimization of viewing angle characteristics. Although TAC has a low in-plane retardation (Ro), its out-of-plane retardation (Rth) is relatively high, which results in low black-white contrast and color shift in liquid crystal displays at wide viewing angles. Thus, it is desirable to reduce Rth of polarizer protective films to improve wide viewing angle performance of LCDs, particularly in-plane-switching (IPS) LCDs.
Birefringence increases with the orientation degree of polymer chains. It is not easy to reduce Rth, however, due to the tendency of in-plane orientation of polymer chains. To overcome this problem, additives with negative intrinsic birefringence have been used to reduce birefringence. For example, Patent Publication Nos. WO 2006098517 and JP 2006096793 disclose a zero-birefringence polymer mixture, wherein an additive with negative intrinsic birefringence is used to compensate the positive intrinsic birefringence of the polymer. Patent Publication No. JP 2006124628 discloses a cycloolefin polymer with low birefringence, but its hydrophobic main chain and side chains are unfavorable to adhere to hydrophilic PVA polarizer. Patent Publications Nos. JP 2004168063 and US 20040099973 also disclose polycarbonate with low birefringence.
Accordingly, there exists a need in the art to provide an optical film with super low or zero retardation to improve wide viewing angle characteristics of LCDs.